Amplifiers introduce nonlinearities in a system due to clipping or crossover distortion. Typically, an amplifier will have an operating range over which an input will produce a linearly proportional output. For many applications, increased efficiency can be achieved if the input is allowed to span a range larger than the linear operating range. Clipping is a nonlinearity which occurs when an amplifier is driven by an input higher than the linear operating range and fails to produce a proportional increase in the output. Crossover distortion is a nonlinearity which occurs when an amplifier is driven by an input below the linear operating range of the amplifier. The nonlinearities may introduce intermodulation products which expand the frequency spectrum of the output outside of an allowable band.
Predistortion is a method of scaling an input in such a way that a subsequent nonlinear amplification results in an output that is linearly proportional to the input. For nonlinearities that do not vary with time, fixed predistortion methods have been used, but many amplifier applications introduce nonlinearities that vary with time due to temperature or operating capacity. For time varying nonlinearities, an adaptive predistortion technique must be applied. In adaptive predistortion, the input is scaled by one of a plurality of scaling factors based on a current condition of the amplifier. As the condition of amplifier changes, the scaling factors are recalculated based on minimizing the deviation of the amplifier output from linear operation. The deviation is an error signal. Typically, the error signal is a very small percentage of the amplifier output. A loss in resolution of the error signal leads directly to reduced benefit from the use of the linearizer. The preferred method of adapting the predistorter is using digital processing at baseband.
Accordingly, there is a need for a method and apparatus for providing a baseband digital error signal in an adaptive predistorter.